1. Field of the Invention
The present invention relates to a method of predicting the remaining lifetime of a metal material and, more particularly, to a method of predicting the remaining lifetime of a metal material which is normally used for a boiler or the like and which suffers creep damage due to being used under high pressure at high temperature.
2. Description of the Related Art
It is well known that, in equipment used under high pressure at high temperature for a long time, as for example in thermal power plant or chemical plant, materials used for components of such actual equipment may suffer creep damage, thereby deteriorating the quality of material while the equipment is in operation. Such deterioration in the quality of material is dominated by such factors as metal temperature, working stress and operating period. In the case of boilers for thermal power plants, it is therefore common practice to determine the quality and size of material to be used by considering those dominant factors, so as to ensure a lifetime normally equivalent to one hundred thousand hours (about fifteen years in the case of normal operation).
However, in such boilers such an accident frequently occurs that the material is damaged in several tens of thousands of hours. It is considered that this accident is caused by an unexpected rise in metal temperature owing to drift, etc. of combustion gas and an abnormal deterioration in the quality of material owing to segregation in the material, e.g., sigma phase embrittlement, etc. Also, the number of power plants which exceed the design lifetime of one hundred thousand hours has recently been increasing. In addition, since an atomic power plant is operated under a base-load condition, the plant is expected to be exposed to severe operating conditions such as an intermediate-load operation and an everyday repetition of start and stop. For these reasons, it has become necessary to develop techniques which enable extension of the lifetime of the plant by exactly predicting the remaining lifetime of the material and proposing the timing of repair and replacement.
Methods of detecting the deterioration in the quality of material are classified into two major types; namely, destructive methods and nondestructive methods. Destructive methods are methods of predicting a remaining lifetime by sampling a portion of a component of an actual equipment, followed by a micrography, a tensile test, a creep test, an impact test and so on, in combination with a stress analysis. The following predicting method using metal structure samples of a material is known as a typical example of a destructive method. Namely, in this method a number of standard metal structures are previously produced under various laboratory conditions and they are compared with a metal structure sample taken from a constituent member of an actual equipment, thereby predicting the lifetime of the material of the member (see, for example, Nippon Kokan Gijutsu No. 62, Pages 531 to 558). An index utilized in this method is the decomposition and agglomeration of a pearlite in the case of Cr-Mo steel while, in the case of stainless steel, the precipitation and agglomeration of carbide in grain boundary and within grain or the state of precipitation of a sigma phase is utilized as the index. For example, there is a technique of predicting the remaining lifetime of stainless steel SUS 321 from the relationship between the quantity of precipitation of the sigma phase and the creep damage in that steel (see Japanese Patent Application Laid-Open Publication No. 201066/83 and Karyoku Genshiryoku Hatsuden Vol. 33, No. 9, Pages 899 to 912). There is another technique of predicting the remaining lifetime from the number of voids produced by creep. (See Zairyo Vol. 28, No. 308, Pages 372 to 378).
The aforesaid prediction method utilizing the quantitative determination of the sigma phase is effective but involves the following problems. Namely, the kind of material which can be handled in this method is limited to stainless steel or high Cr steel, and the state of precipitation of the sigma phase varies even in the same stainless steel owing to a slight difference in chemichal composition. Further, a prediction method utilizing the quantitative determination of a creep cavity is effective but involves also the following problems. Namely, the kind of material which can be handled in this method is limited to a material of low transgranular ductility, such as stainless steel or high Cr steel (for example, HK 40), so that it is difficult to apply this method to a material of high transgranular ductility such as low alloy steel for boilers, because creep cavities are difficult to be formed in that material.
Nondestructive methods are methods of indirectly predicting a remaining lifetime by detecting change in a metal structure, such as the decomposition owing to heating or creep, and physical change owing to formation of voids.
In this case, various kinds of physical quantities are available, and the following items have already been put into practical use or under study: namely, for example, electrical resistance (Japanese Patent Application Laid-Open Publication No. 60248/83), ultrasonic sound speed (Japanese Patent Application Laid-Open Publication No. 120585/78), and misorientation by X-ray and coil impedance by eddy current (Japanese Patent Application Laid-Open Publication No. 88781/78).
The nondestructive methods generally involve the following problems. Firstly, a high-precision device is needed since extremely slight changes must be detected in order to detect variations in physical quantities such as electrical resistance which might be caused by microscopic changes in the structure of a metal material. Further, there is a possibility that a large error may occur due to the handling of the device, the measurement environment and so on. In particular, unlike turbines, boilers are commonly placed in an adverse measurement environment, and this makes it difficult to perform accurate measurement. In addition, turbines are generally made of a material, such as Cr-Mo-V steel, having a high carbon content, so that the physical quantities such as electrical resistance decrease to a significant extent. On the other hand, since boilers are made of a material, such as 2.multidot.1/4 Cr - 1 Mo steel, having a low carbon content, the physical quantities don't greatly decrease, and this makes it difficult to perform satisfactory evaluation. Secondly, in these nondestructive methods, a master curve is previously produced under laboratory conditions and it is compared with the result of measurement of a component of an actual equipment to predict the remaining lifetime of that component. However, physical quantities to be measured are obtained by detecting extremely slight changes in a material, and the absolute values of the physical quantities vary in compliance with changes in an initial state of the material or merely in heating conditions. For this reason, the degree of damage must be evaluated on the basis of a relative value between the physical quantity of a damaged material and that in the initial state of the material before damage or in a merely heated state, not on the basis of the absolute value of the physical quantity. Therefore, when the master curve is to be produced under laboratory conditions, it is necessary to use a material the initial state of which is the same as that of a component of a actual equipment whose lifetime is to be measured, that is, to use a material of the same charge. In the present circumstances, however, it is impossible to obtain the materials from which boilers currently in operation (in particular, boilers produced ten or more years ago) were produced. Moreover, the data for structure, hardness and short-time tensile test might be obtained as the data of the material at that time, but it is virtually impossible to obtain the data for various physical quantities to be utilized in evaluation of the lifetime. Further, since previous methods of producing materials at that time differ from current methods, it is very difficult to reproduce the materials at that time.
Accordingly, the aforesaid prior art further involves the following problems.
In the destructive methods, it is the most reliable method to predict a remaining lifetime by subjecting a sample taken from a component of an actual equipment to a creep test. However, this method needs enormous costs and time and, in addition, a range inspected by this method is restricted. Also, in the method utilizing structure observation, rapid evaluation is enabled, but quantitative evaluation is difficult to perform since the evaluation is conducted by comparison with standard photomicrographs.
In addition, the change in structure is dominated mainly by temperature and time, but the effect of stress is small. For this reason, it is difficult to directly associate the creep damage with the change in structure.
In the nondestructive methods, there is a possibility that an error may occur because extremely slight changes in physical quantity of a material used are measured, and since the physical quantity varies due to environmental temperature etc., it is difficult to strictly compare the physical quantity with a master curve produced under laboratory conditions. In particular, it is impossible to obtain the data for materials produced ten or more years ago, and it is also difficult to reproduce such materials. Accordingly, both the destructive and nondestructive methods involve a number of problems.